1. Field of the Invention
The Present invention relates to an oxide high-temperature superconducting material, more particularly to an oxide high-temperature superconducting powdery or crystalline material having an excellent orientability, a superconducting wire made from the powdery or crystalline material and having an improved critical current density, and a method of preparing the material and a method of preparing the wire.
2. Related Art
Applications of superconductive materials are in general classified into two fields: a heavy electric engineering field suitable for high current and strong magnetic field; and a light electric engineering field suitable for low current or low voltage.
Most of prior superconductors are not practically applied. As materials for superconducting wires are known intermetallic compounds such as Nb.sub.3 Sn, Nb.sub.3 Ge, a Nb-Ti alloy and etc. These compounds have a low temperature of exhibiting superconductivity, socalled critical temperature Tc. The highest critical temperature is 23 K for Nb.sub.3 Ge. Liquid helium (4.2 K) is needed for cooling Nb.sub.3 Ge.
Recently, oxide superconducting substances having a high critical temperature Tc have been found, for example, a La-Sr-Cu oxide (35-40 K) and Y-Ba-Cu oxide (90-100 K) and Bi-Sr-Ca-Cu oxide (70-100 K). The critical temperature of this Y-Ba-Cu oxide, Ba-Sr-Ca-Cu oxide and Tl-Ba-Ca-Cu oxide are much higher than the temperature of liquid nitrogen (77 K). Thus, expensive liquid helium used for cooling the prior intermetallic compounds is not necessary to use for cooling the Y-Ba-Cu oxide, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu. That is, the Y-Ba-Cu, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu oxide can exhibit superconductivity with inexpensive liquid nitrogen. Therefore, the Y-Ba-Cu, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu oxide has been noted as practical superconducting substances and demanded to be put into practical use. As high-temperature superconducting substances having a much higher superconducting transition temperature than that of prior superconducting substances, a lanthanum-barium-copper oxide was discovered by Dr. J. G. Bednorz and Dr. K. A. Mdller in the beginning of 1986 (see Z. Phys. B Condesed Matter 64, 1986, pp. 189-193), and then an yttrium-bariumcopper oxide (referred to as Y-Ba-Cu oxide) discovered by Dr. Chu of Houston University, U.S.A., in the spring of 1987 (see Physical Review Letters, Vol. 58, No. 9, 1987, pp. 908-910) and at the same time.also in Japan (see Japanese Journal of Applied Physics, Vol. 26, No. 4, 1987, pp. L314-L315). Now intensive researches and developments have been made on a basic science about compositions, crystal structures, properties and theory of the superconducting substances, on the synthesis and applications to heavy or light electric engineering fields of superconducting substances, and further on survey of materials having superconductivity at a higher temperature, e.g., room temperature.
Among techniques to be researched and developed, a technique of making wires from the superconducting substances is important as an elemental technique in applications to the heavy electric engineering for superconductive magnets or etc.
In one of the most general methods of making wires or ribbons from Y-Ba-Cu, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu oxide, a metal sheath is filled with an Y-Ba-Cu, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu oxide powder and worked by a swaging machine or drawbench to be formed in the wire or by a roll machine to be formed in the ribbons. The wires or ribbons are fired at about 900.degree. C. for a few hours to sinter the Y-Ba-Cu oxide powder which otherwise will not form any current path therein, thereby allowing the particles to diffuse in each other so that the current path can be formed. In case of Bi-Sr-Ca-Cu-O firing is preferably performed at about 1,845.degree. C.; and in case of Tl-Ba-Ca-Cu-O firing is preferably performed at about 835.degree. C.
The thus obtained superconductors has a perovskite type layer crystal structure. This structure of the Y-Ba-Cu oxide superconductor is schematically illustrated in FIG. 2 attached hereto. In FIG. 2, 1 is yttrium, 2 barium, 3 copper, 4 oxygen and 5 oxygen (vacant). Electric current flows on the layer of crystal, that is, electron can easily flow on the a axis-b axis plane of crystal (see Journal of the Japan Metal Society, Vol. 26. No. 10, 1987, p. 971).
Therefore, it is important to align crystals so that the a axis-b axis planes of the crystals orient to the longitudinal direction of the wire. Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O have similar crystalline structures to that of Y-Ba-Ca-Cu-O. In order to attain a high Jc, it is necessary to orient a axis-b axis face.
From the viewpoint of this orientation, a melt-quenching method (see Symposium of Superconducting Substance Chemistry, October, 1987) and a chemical vapor depositing method (see Japanese Patent Application No. 57-118002) have been studied on. For example, a thin superconducting film having an excellent orientability has already been obtained by the chemical vapor depositing method. Such thin superconducting film has a high critical current density Jc of 10.sup.3 A/cm.sup.2 generally required. However, this method cannot produce a long film or wire.
The metal superconducting material is relatively easily drawn in a wire but, on the other hand, the oxide superconducting material is poor in ductility and hardly formed in wires. Therefore, for making wires from the oxide superconducting material it is necessary to fill a metal pipe with a powder of the oxide superconducting material, draw the pipe and heat treat the drawn pipe to sinter the oxide superconducting material. However, as mentioned above, this oxide superconducting material taking the perovskite type crystal structure is anisotropic in the electric current-flowing direction. Furthermore, since this oxide has a layer structure, the crystal particles are in a plate form and hardly aligned in a direction as compared with the thin film. Therefore, current hardly flows between the particles, which prevents the critical current density Jc from being raised up. Furthermore, the orientation of crystals has not been taken into account in prior art. Therefore, crystal grains of the Y-Ba-Cu oxide grow at random and they are not oriented, so that low critical current density is obtained.
The critical current density at the temperature of liquid nitrogen (77 K) of the Y-Ba-Cu oxide produced by prior methods is now about 2,000 A/cm.sup.2 as reported in Nikkan Kogyo Shimbun dated October 7, 1987.